How do I give a completely black object a glow effect?
It looks like you want Eevee for easy bloom?
You can repurpose an edge lining technique for it.  Give your object two materials and a solidify modifier with a material offset.  Make one material black, and for the other, give it emission for backfaces, but transparency for front faces:

Note that you'll need to enable some sort of blending (alpha clip is fine) and make sure you're not culling bakfaces, in material settings as shown.
There are a number of additional ways you could achieve this, constraint-placed shadowing meshes, render layers, and probably others.  But this is previewable, simple enough, and should get you what you're after.